What is Coherent Optics

What is Coherent Optics?

At its most basic, coherent optical transmission is a technique that uses modulation of the amplitude and phase of the light, as well as transmission across two polarizations, to enable the transport of considerably more information through a fiber optic cable. Coherent optics in fiber optic communications have been around almost as long as fiber optics. They were first considered for long haul fiber optics in the late 1980s to extend optical repeater distance (due to the much higher sensitivity obtained with coherent receivers).

Using digital signal processing at both the transmitter and receiver, coherent optics also offers higher bit-rates, greater degrees of flexibility, simpler photonic line systems, and better optical performance.

Coherent optics solves the capacity problems network providers are facing. It takes the typical ones and zeroes in a digital signal—the blinking on and off of the light in the fiber—and uses sophisticated technology to modulate the amplitude and phase of that light and send the signal across each of two polarizations. This, in turn, imparts considerably more information onto the light speeding through a fiber optic cable.

Coherent optics provides the performance and flexibility to transport significantly more information on the same fiber. And while coherent optics is a far more complex technology, it has enabled us to increase the amount of information each laser can transmit over fiber optics by 80 times – from 10 gigabits per second to 800 gigabits per second. And of course, this technology continues to evolve.

Coherent Optical Technology

Recently interest in coherent optics has resurfaced for long haul transmission due to the high spectral efficiency that coherent optics enable, as well as larger link budgets.

Coherent optical technology forms the foundation of the industry’s drive to achieve transport speeds of 100G and beyond, delivering Terabits of information across a single fiber pair. Digital signal processors electronically compensate for Chromatic and Polarization Mode Dispersion (CD and PMD) to enable robust performance over old and new fibers alike, and eliminate the need for dispersion-sloped compensating modules from the photonic line. Coherent optics enables greater network flexibility and programmability by supporting different baud rates and modulation formats. This results in greater flexibility in line rates, with scalability from 100G to 400G and beyond per single signal carrier, delivering increased data throughput at a lower cost per bit.

Key Attributes of Coherent Optics

Advanced coherent optical technology has a number of key attributes, including:

High-gain soft-decision Forward Error Correction (FEC):

Enables signals to traverse longer distances while requiring fewer regenerator points. It provides more margin, allowing higher bit-rate signals to traverse farther distances. This results in simpler photonic lines, less equipment, and lower costs—while, of course, increasing bandwidth significantly. e of all available capacity and convert excess margin into revenue-generating services.

Strong Mitigation to Dispersion:

Offers better optical performance at higher bit-rates. Coherent processors must account for dispersion effects after the signal has been transmitted across the fiber, including compensating for CD and PMD. The advanced digital signal processors in coherent optics take away the headaches of planning dispersion maps and budgeting for PMD by mitigating these effects. Additionally, coherent processors improve tolerances for Polarization-Dependent Loss (PDL) and must rapidly track the State of Polarization (SOP) to avoid bit-errors due to cycle slips that would otherwise affect optical performance. As a result, operators can deploy line rates up to 400G per carrier across longer distances than ever; high bit-rate signals can even be deployed on old fiber that previously couldn’t support 10G.

Programmability:

It means the technology can be tailored for a wide variety of networks and applications and the same card can support multiple modulation formats and/or different baud rates, enabling operators to choose from a variety of line rates. Fully programmable coherent transceivers provide a wide range of tun ability options with fine granularity between incremental capacities, enabling network operators to make us

Spectral Shaping:

Provides greater capacity across cascaded Re-configurable Optical Add-Drop Multiplexers (ROADMs), resulting in increased spectral efficiency for super channels. Spectral shaping is critical in flexible grid systems because it allows carriers to be squeezed closer together to maximize capacity.

Applications of Coherent Optics for PON Systems

All PON systems and most optical transport systems use direct detection optics, that is the optical signal from the transmitter traverses the optical fiber and lands directly on the photo diode which converts the optical signal to electrical current.

Coherent optics is a technology that can drastically increase optical receiver sensitivity and easily meet and surpass the highest link budgets defined in the standards. However coherent optics has typically been considered prohibitively expensive for use in consumer grade optics due to the large number of precision optical components required for the receiver and the ultra-stability required for the laser transmitter.

Why Should Coherent Optics Matter to You?

Recently interest in coherent optics has resurfaced for long haul transmission due to the high spectral efficiency that coherent optics enable, as well as larger link budgets.

Coherent optics use classic homo dyne receiver technology invented for the radio communications. They have the advantage of greatly increasing the sensitivity of a receiver while allowing very selective tuning, which means that the channel spacing can be very close, whether they are radio or optical channels. Already coherent optics are increasing the capacity of installed long haul fiber networks and are partly responsible for reducing the cost per bit for high speed communications, which our modern society has come to depend on.